The present invention relates to a novel flame-retardant resin composition free from halogen or, more particularly, relates to a novel flame-retardant thermoplastic resin composition without containing any halogen compounds but still having excellent mechanical properties and safety against pollution of environments as well as sufficiently high flame retardancy even by compounding a decreased amount of non-halogen flame-retardant agent such as metal hydroxides.
As is well known, one of the most serious problems in thermoplastic resins is the inflammability of the articles shaped or molded therefrom, which sometimes prohibit use of the resin in certain applications requiring high flame retardancy of the resin articles. While various technological proposals and attempts have been made heretofore to impart flame retardancy to thermoplastic resins, a most conventional way in this regard is to prepare a flame-retardant thermoplastic resin composition by compounding the resin with flame-retardant agents including a halogen-based compound, e.g., bromine compounds, and antimony oxide in combination. While halogen-based flame-retardant agents can impart excellent flame-retardancy to the resin composition by way of their radical-trapping activity and evolution of nonflammable gases, such halogen-containing gases are generally very toxic against human body and the damage in a fire hazard is sometimes multiplied by a large volume of such toxic gases evolved from thermoplastic resin articles containing a halogen-based flame-retardant agent. Accordingly, it is eagerly desired to develop a flame-retardant thermoplastic resin composition without formulation of a halogen-based flame-retardant agent.
A proposal recently made in this regard for polyolefin-based resin compositions, which are widely employed as an insulating material or sheath of electric wires and cables, is for the use of a metal hydroxide such as magnesium hydroxide as a non-halogen flame-retardant agent. Metal hydroxides are mostly advantageous as a flame-retardant agent in respect of their low toxicity and small volumes of smoke evolution by combustion along with little corrosiveness. The flame-retardant effect on the matrix resin thereby is exhibited by means of the water of crystallization released from the resin articles under combustion. The effect of flame retardancy exhibited by this mechanism, however, cannot be high enough and the oxygen consumption index, referred to as the OI hereinafter, can rarely exceed 25 even in a resin composition containing 100 parts by weight of a metal hydroxide powder per 100 parts by weight of the polyolefin resin.
An alternative proposal is made in Japanese Patents Nos. 2825500, 3019225 and 3072746, according to which the flame retardancy of a polyolefin resin composition can be improved without adversely affecting the mechanical properties of the resin articles by the use of a metal hydroxide powder after a surface treatment with a special surface treatment agent as the flame-retardant agent. Japanese Patent Publication 7-119324, Japanese Patent Nos. 3051211 and 3063759 and Japanese Patent Kokai 8-295784 further disclose a flame-retardant resin composition which is prepared by compounding a polyolefin resin with a specific metal hydroxide and a specific organopolysiloxane compound in combination. Each of these methods is in fact effective to improve the flame retardancy of the resin composition prepared according to the proposed formulation so that the amount of the metal hydroxide can be substantially decreased without affecting the mechanical properties of the resin articles prepared from the resin composition. The effectiveness of these methods, however, is still not high enough so that debut of a flame-retardant thermoplastic resin composition having excellent flame retardancy without a decrease in the mechanical properties is eagerly desired.